FIG. 1 is a view showing a conventional polarizing beam splitter.
When light having a P-polarization and an S-polarization in a mixed state is incident upon a polarizing beam splitter (PBS) 1, the P-polarization is transmitted through the polarizing beam splitter 1 and the S-polarization is reflected by the polarizing beam splitter 1.
The reflected S-polarization and the transmitted P-polarization are directed in the same direction by diamond-shaped prisms 2 and 3.
For example, the P-polarization is transmitted through the prism and is then changed into an S-polarization by a half wave plate (retarder) 4.
As a result, the light having the P-polarization and the S-polarization in the mixed state is changed into the same polarization, e.g. the S-polarization, by the polarizing beam splitter. That is, the light having the P-polarization and the S-polarization in the mixed state has the same direction.
An operation principle of a stereoscopic image apparatus using the conventional polarizing beam splitter is as follows. U.S. Pat. No. 7,857,455 is referred to.
As shown in FIG. 2, light emitted from an image surface 5 generating an image in a projector passes through a projection lens 6 and is then split into two beams by a polarizing beam splitter 7.
That is, light having an S-polarization state and a P-polarization state is reflected by the polarizing beam splitter 7 or transmitted through the polarizing beam splitter 7.
The transmitted or reflected P-polarization component is changed into S-polarization while passing through a half wave retarder 8. The S-polarization is concentrated on a projection screen via reflective members 9 and 10, a polarizer 11, and a modulator 12.
The modulator 12 may change a polarization state/direction, for example, according to an electric signal.
On the other hand, the S-polarization component reflected by the polarizing beam splitter 7 reaches the projection screen via a reflective member 13 in a state in which the S-polarization is maintained in the same direction.
Consequently, the light, having mixed polarization states/directions, emitted from the image surface 5 is changed into a single S-polarization.
However, the stereoscopic image apparatus using the conventional polarizing beam splitter has the following problems.
In general, a vertical exit angle of the projector is about 15 degrees. A case in which the exit angle is 15 degrees is shown in FIG. 3. A polarizer and a modulator are omitted from FIG. 3 for simplicity's sake.
It is assumed that the distance between a polarizing beam splitter and a reflective member 16 and the distance between the polarizing beam splitter and another reflective member 16 are h1 and h2, respectively, and the distances between the respective reflective member 16 and 17 and a screen 18 are L1 and L2, respectively.
In this case, an angle θ1 between the light reflected by the reflective member 16 and an optical axis of the light emitted from the projector is TAN−1(h1/L1) and an angle θ2 between the light reflected by the reflective member 17 and the optical axis of the light emitted from the projector is TAN−1(h2/L2).
Reference numeral 161 indicates the light reflected by the reflective member 16 and reference numeral 171 indicates the light reflected by the reflective member 17.
Distortion of an image on the screen 18 due to the angles θ1 and θ2 is as follows. FIG. 4 is an enlarged view showing part (A) of FIG. 3.
Referring to FIG. 4, reference numeral 161 indicates the light reflected by the reflective member 16 and reference numeral 171 indicates the light reflected by the reflective member 17.
In addition, reference numeral 162 indicates an image-forming surface of the light reflected by the reflective member 16 and reference numeral 172 indicates an image-forming surface of the light reflected by the reflective member 17.
On the assumption that the height of the screen 18 is H, a height difference d1 between the image-forming surface of the light reflected by the reflective member 16 and the image on the screen 18 and a height difference d2 between the image-forming surface of the light reflected by the reflective member 17 and the image on the screen 18 are expressed as follows.d1=H TAN(θ1), d2=H TAN(θ2)
Consequently, the beams reflected by the reflective members 16 and 17 form images on the image-forming surface with a distance difference Δ=(H/2) {TAN (θ1)+TAN (θ2)}.
In a case in which h1≈h2=340 mm, L1≈L2=15000 mm, and H=8500 mm, θ1≈θ2=1.3 degrees and, therefore, Δ=193 mm.
This means that the light reflected by the reflective member 16 and the light reflected by the reflective member 17 deviate from each other on the image-forming surface by a maximum of 193 mm. In general, the spot size of light is several mm. As the distance from the center of the screen 18 is increased, therefore, the image is less visible, which leads to limitations in use.